In semiconductor manufacturing devices, it is necessary to precisely determine the spatial position of certain movable objects such as traversing tables. To that end, usually position-measuring systems are used, which make position information available on the output side. A computer-controlled sequencing control is possible in these devices based on the position information ascertained. Thus, for example, in wafer steppers, the position of mask and wafer must be measured very precisely in all 6 degrees of freedom (6DOF). In conventional methods, this position measuring is mainly carried out using several laser interferometers. The relative position of movable components with respect to a so-called metrology frame is determined in a conventional manner via the laser interferometers. In the future, it must be assumed that the stringent accuracy requirements of the position measuring, accompanied at the same time by increasing traversing speeds of the various parts, will further increase. While, for instance, until now, accuracy requirements of a few nm at speeds of approximately 1 m/s are specified, in the future, required subnanometer accuracies at markedly higher speeds must be assumed. However, given such high accuracy requirements, laser interferometers can no longer be used as position-measuring systems, since the refractive-index fluctuations in the ambient air, even given optimal air conditioning, lead to unacceptable measured-value fluctuations of several nm in the position measuring.
For this reason, alternative position-measuring systems have already been proposed for devices of this type. Thus, it is described, for instance, in European Published Patent Application No. 1 019 669 to use optical position-measuring systems having so-called cross gratings as a two-dimensional (grating-)measuring graduation, that is, grating-based position-measuring systems. Systems of this type are scarcely influenced by possible fluctuations in the refractive index of the air, and therefore allow easily reproducible position measurements.
To determine the position of movable objects, i.e., thus, for instance, of traversing tables, in all degrees of freedom, a plurality of measured quantities (at least 1 measured value per degree of freedom) that are independent of each other must be determined simultaneously for that purpose. To that end, generally a plurality of scanning units of a position-measuring system are mounted at various positions on each traversing table.
In that context, a single scanning unit is in each case able to measure one axis independent of the other scanning units, or also several axes in combination. In this configuration, in which the measuring graduation of the position-measuring system is secured to the motionless part of the machine (metrology frame), the scanning units must be mounted on the moving traversing tables, and the measurement data of the scanning units must be transmitted to a stationary sequential electronics via a cable trailing device. At the same time, it is necessary for the dynamic performance of the machine to keep the moving masses and the dynamic forces introduced by the cable trailing device in response to movement of the traversing table as small as possible. Particularly in highly dynamic and at the same time highly precise applications, very high requirements arise for the smallest possible masses and unit volume. Because of the demand for a highly precise measurement of the position in these applications, at the same time, very great demands result for a distortion-free and interference-free transmission of the position-specific measurement data to a sequential electronics often far away (up to >10 m) from the traversing table. The individual scanning-unit data are processed by the sequential electronics to obtain a position of the traversing table in all degrees of freedom and made available to a sequencing and motion control.
The motors for driving the traversing table are situated at or in the immediate vicinity of the traversing table. Strong electromagnetic fields are thereby formed which, in particular, can falsify analog measuring signals that are transmitted on long electrical lines. On the other hand, robust, interference-immune line drivers in the individual scanning units and adequately shielded signal lines from each scanning unit to the sequential electronics disadvantageously influence the mass and the unit volume of the scanning unit and traversing table. Because the scanning units are mounted on the traversing table, additional supply lines and signal lines are necessary in the cable trailing device between the traversing table and the stationary components, these lines disadvantageously influencing the inert mass and stiffness of this cable trailing device, as well.
It is necessary to keep the power loss, introduced at the traversing table by the scanning units and the signal transmission, as low as possible, in order to minimize heating of the traversing table or parts thereof, since this results directly in a measuring error.
Therefore, even when using grating-based position-measuring systems, a number of problems result which must be solved in order to ensure a sufficiently accurate determination of position.